This invention relates to the ecologically safe disposal of intermittent burnable contaminants in gaseous streams. More particularly, the present invention relates to the manner of handling the gaseous emissions from the regeneration of a fixed bed dehydrogenation catalyst.
Non-oxidative dehydrogenation of organic compounds, particularly hydrocarbons, has been in use for years. These processes are generally applied to C.sub.4 to C.sub.8 hydrocarbons and particularly for the preparation of less saturated hydrocarbons having four or five carbon atoms, for example, n-butenes and butadiene prepared from n-butane.
The dehydrogenations are carried out over a contact mass, usually by passing a gaseous stream containing a dehydrogenable hydrocarbon over the mass in a reactor. The contact mass is preferably a material which is catalytic for the dehydrogenation, for example a metal oxide or mixture of oxides such as chromia-alumina. These catalysts may be supported or unsupported. A consideration in selecting the catalyst form is the pressure drop through the reactor. It can be appreciated that the catalyst particles must have sufficient surface area to provide a catalytic benefit, but nonetheless not be of too fine a configuration such that there is a large pressure drop through the reactor. A general range of catalytic particle sizes which accommodates both objectives is about 1/16 to 1.
The non-oxidative dehydrogenations are preferably carried out in a cyclic manner, wherein a gaseous feed containing the dehydrogenable compound, for example butane, is passed through a catalyst bed for a period of time. During this passage, the compound is converted to a less saturated compound, e.g., butene and butadiene, and the catalyst bed becomes coked to some extent. Basically the process involves stopping the dehydrogenation feed and initiation of a regenerative gaseous feed through the catalyst bed. In the case of a hydrocarbon, such as butane, the coking of the catalyst is common and an oxidizing gas, i.e., one containing molecular oxygen such as air is used to regenerate the catalyst. The oxygen aids in burning off or other wise removing the carbonaceous material which has become deposited on the catalyst during the dehydrogenation cycle. Although these are the two basic portions of the cycle, there may be other steps such as purges, evacuations, and the like.
Generally, several reactors are operated as a unit to have some reactors dehydrogenating while other reactors are in regeneration, purge, evacuation, or the like. Thus, in this manner the process equipment for handling product, regeneration gases, etc., is utilized most effectively. The problem to which the present invention is particularly directed is in the nature of the off gases from the regeneration step.
It has long been recognized that the regenerative gases passing through the reactor are not constant in composition for the entire duration of the regeneration. The initial portion of the regenerative gases are generally quite high in burnable materials. This was noted by early workers such as J.R. Bates in U.S. Pat. No. 2,285,401. Similarly, it was noted that this initial portion of the regenerative gases was deficient in oxygen, due probably to some combustion of the material coked on the catalyst. The patentee determined that the regenerative off gases, because of their low oxygen content, would be good process gases to add to the feed stream during the dehydrogenation cycle. To improve the regenerative gases for this employment, they could be subjected to purification and controlled combustion in catalytic combustion zone. The purpose being as much to remove oxygen as the burnable materials, hence oxygen was excluded from the catalytic combustion zone. This undoubtedly resulted in very poor removal of burnables from the gaseous stream.
The problem faced by the patentee was the need for a feed diluent, one which was available in large supplies and did not contain reactive materials, detrimental to the reaction. The regenerative off gas, which was low in oxygen, met these requirements, and if the oxygen content were too high the gas could be burned further to leave it practically oxygen free.
The problem faced now is not the need for a feed stream diluent, but a means to totally destroy or convert the burnable materials in the initial portion of the regenerative gases into non-polluting materials. Various proposals have been made and each is objectionable or unworkable in practice. For example, attempts were made to send all of the regenerative off gases into an unfired waste heat boiler, wherein by dent of the conditions therein the burnables would undergo combustion; however, the combustion has been controllable and explosive conditions have occurred. Another suggestion has been to feed the gases to a fired boiler. This has proven unworkable because of the great variation in the constitution of the regenerative off gases, not only over the duration of a particular cycle wherein the gases coming out of the reactor have a high initial burnable content and decline thereafter, but also variations from cycle to cycle as to the magnitude of the initial burnables and the rate of change of the composition of the off gases. Burners characteristically are designed and built for constant state operation. Fluctuations such as those imparted by feeding in the regenerative off gases are not tolerable and at best would be the cause of frequent flameouts. Similarly, boilers are tightly designed and no performance could be depended upon with the variations the regenerative off gases could cause.
Moreover, none of the alternates tried were readily controllable to the extent necessary to essentially destroy the waste materials in the regenerative gases.
The present invention features both process and apparatus for the removal of high percentages of the burnable materials in the regenerative off gases.